Diesel multiple unit

A diesel multiple unit or DMU is a multiple unit train consisting of multiple carriages powered by one or more on-board diesel engines. They may also be referred to as a railcar or railmotor depending on country.

Design

The diesel engine may be located above the frame in an engine bay or under the floor. Driving controls can be provided at each end of the car, at one end only, or totally omitted, giving pure 'trailer' cars that still have motors which rely on cab cars for control.

Types

DMUs are usually classified by the method of transmitting motive power to their wheels.

Diesel mechanical

In a diesel mechanical multiple unit the rotating energy of the engine is transmitted via a gearbox and driveshaft directly to the wheels of the train, much like a car. The transmissions can be shifted manually by the driver, but in most applications gears are changed automatically.

Diesel hydraulic

In a diesel hydraulic multiple unit, a hydraulic torque converter, a type of fluid coupling, acts as the transmission medium for the motive power of the diesel engine to turn the wheels. Some units feature a hybrid mix of hydraulic and mechanical transmissions, usually reverting to the latter at higher operating speeds as this decreases engine RPM and noise.

Diesel Electric

In a diesel electric multiple unit (DEMU) a diesel engine drives an electrical generator which produces electrical energy. This current is then fed to electric traction motors on the wheels or bogies in the same way as a conventional diesel electric locomotive.

In most modern DEMUs, each car is entirely self-contained and has its own engine, generator and electric motors. In older designs some cars within the consist may be entirely unpowered or only feature electric motors, obtaining electrical current from other cars in the consist which have a generator and engine.

Direct-drive diesel locomotives often require an impractical number of gears to keep the engine within its powerband; coupling the diesel to a generator eliminates this problem. Power still needs to be transmitted to the generator or alternator via a simple gearbox but this is advantageous because;

Mere cables transmit the power to the axle traction motors, not a complex system of rods

Sending power to a generator via a low powerband (i.e. diesel) is analogous to sending power to a propeller, in that one gear ratio is enough, unlike sending power to wheels, where several ratios are necessary. This is because the high friction between wheels and contact surface requires a low gear for takeoff to avoid stalling, and higher gears thereafter, since the output shaft moves relative to vehicle speed and would over-rev the engine if just one gear was available. In a generator or alternator, the force is magnetism, not a high friction surface so a high gear is fine since the magnetism is not great enough to stall the engine. The result is that the diesel-electric system puts the diesel’s low powerband to best use.

Rheostatic braking (sometimes called dynamic, electric or regenerative braking) is also available on some DEMUs. By attaching the motors to an electrical load they act as generators and slow the train down. The load is usually resistors which dissipate electrical energy as heat to the atmosphere, though it is sometimes used to heat the interiors. In some cases it is stored for later use or passed back into external power supplies. The advantage for this method of braking is reduced wear and tear on the main service brakes.

Benefits

A train composed of DMU cars scales well as it allows extra passenger capacity to be added at the same time as motive power. It also permits passenger capacity to be matched to demand, and for trains to be split and joined en-route. It is not necessary to match the power available to the size and weight of the train- each unit is capable of moving itself, so as units are added, the power available to move the train increases by the necessary amount.

Distribution of the propulsion among the cars also results in a system that is less vulnerable to single-point-of-failure outages. Many classes of DMU are capable of operating with faulty units still in the consist. Because of the self contained nature of diesel engines, there is no need to run overhead electric lines or electrified track, which can result in lower system construction costs.

Around the World

Australia

DMUs were first introduced to Australia in the early 20th century for use on quiet branchlines that could not justify a locomotive hauled service. Today a range of modern DMUs are extensively used across Australia for both commuter and intercity routes:

Western Australia's Transwa operates the Prospector, Avonlink and Australind on medium and long country services.
In Victoria V/Line operate Sprinter and VLocity DMUs on both commuter and medium distance country services.
In New South Wales CountryLink uses Xplorer DMUs on country services, and CityRail operate the 620 / 720 class, Endeavour, and Hunter railcars on commuter services.
A DMU variant of Queensland Rail's QR Tilt Train is in use on the long distance Brisbane to Cairns service.
TransAdelaide in South Australia use 3000 class and 2000 class DMUs on their suburban rail lines.

Germany

The Flying Hamburger of Germany, introduced 1933, established the then-time fastest regular railway connection of the world. Top speed was 160 km/h, the average speed being 124 km/h on the tracks between Berlin and Hamburg.

The Trans Europ Express travelled international traffic between countries like Germany, France, Italy, Netherlands in the 1950s and 1960s. They were diesel multiple units since the electrical systems varied a lot.

Also from Germany is the CargoSprinter concept. With two motorized units and three flatbed wagons between them, this DMU container train tried to compete with container road trucks by profiting from existing railway access to factories and businesses, but eliminating the need for inflexible locomotive-pulled cargo trains. The payload was 160 tons. Suffering from technical problems and failing political support for short-haul cargo railway connections, the prototypes were sold to Austria.

Currently DMUs are used in large numbers for local traffic (e.g. DB Class 642) and fast, tilting regional traffic (e.g. DB Class 612). The introduction of DB Class 605 as a diesel variant of the tilting EMU DB Class 411/415 in 2001 was not successful.

Ireland

In the Republic of Ireland the Córas Iompair Éireann (CIE), which controlled the republic's railways between 1945-86, introduced DMUs in the mid-1950s and they were the first diesel trains on many main lines. The Great Northern Railway, merged into CIE at this time, also brought some DMUs of its own of a similar style. They were not well suited to the long-distance tasks and were replaced in a few years by traditional trains with new diesel locomotives, being then mostly restricted to Dublin commuter lines. The power systems were worn out by the 1970s so they were converted to normal carriages pulled by diesel locomotives on these suburban routes, and DMUs disappeared from CIE. But since 1987, Iarnród Éireann (IE) have been increasing the use of this type of train, in order to replace older locomotives and carriages, using new types of train manufactured in a number of overseas countries.

In Northern Ireland the Northern Ireland Railways used DMUs extensively from the 1950s throughout its system, notably the NIR 80 Class introduced in the early 1970s and currently being replaced (as of 2007).

Japan

The development of DMUs in Japan started in 1950s following the improvement of fuel supply that was critical during World War II. In 1953, the Japanese National Railways put the hydraulic torque converter into practical use. This invention facilitated the development of DMUs, which spread all over the nation as not only on local services but also on long distance express services. In 1960, the 9-car KiHa 80 series DMUs debuted on Hatsukari limited express service connecting Tokyo and Aomori (about 750 km).

Presently, all the passenger railway companies of Japan Railways Group (JR Group) operate DMUs on semi-trunk and local lines while almost all of trunk lines have been electrified. On the JR lines not yet electrified, locomotive trains were abandoned but only small number of sleeper trains. There are also a number of independent railway lines that operate DMUs.

North America

File:Tri-rail dmu.jpg

South Florida Regional Transportation Authority Tri-Rail Bi-Level DMU.

Two Bombardier Talent low-floor DMUs on the O-Train prototype line in Ottawa Canada

One of the first Diesel Multiple Units in North America was the Budd Rail Diesel Car (RDC). The RDC was a single passenger car with two diesel engines and two sets of controls. Any number of cars could be connected together and all the propulsion systems controlled from a single operator's station or cab. Introduced in the early 1940s, the cars were used on rural railway lines that did not warrant full passenger trains, or short commuter services.

In the United States of America only heavyweight DMU systems are permitted on freight rail corridors. This is due to the Federal Railway Administration setting higher buff strength requirements than European regulators, effectively prohibited the use of lightweight European-style light-rail DMUs on main line railways.

Several rail operators in the United States use DMUs suitable for mainline use:

The South Florida Regional Transportation Authority has used a Colorado Railcar bi-Level DMU and Coach consist since October 2006 as a technology demonstrator on the 87-mile long Tri-Rail commuter rail line between Miami and West Palm Beach, Florida. In early 2007, three more bi-level DMUs and an additional bi-level non-powered coach were acquired.

The Florida Department of Transportation is currently conducting preliminary engineering for a DMU commuter line connecting Orlando, Florida with neighboring communities north and south of its downtown, with service targeted to begin in 2009.[1]

The Los Angeles County Metropolitan Transportation Authority approved an allocation of $250,000 for a feasibility study of DMUs for "future transportation options for the region" on July 5, 2006 (Ara Najarian, Metro Board Member).

Chicago's commuter rail line, Metra, is studying the use of DMUs on its newly proposed lines (STAR line, SES). They claim these DMUs will have better acceleration, be more fuel efficient, and seat more customers than the current diesel engine cab and double decker rail cars that are currently in use. [2]

Amtrak is reportedly encouraging the state of Vermont to buy DMUs for Amtrak's state-subsidized Vermonter service, in order to save money over the current locomotive-pulled arrangement. [3]

Trimet, the public transport operator for the Portland, OR metropolitan area will use 3 DMUs to operate WES, or West Side Express, a regional rail line that will run half-hourly during rush hours between Beaverton and Wilsonville starting fall, 2008. These are single level cars built by Colorado Railcar.

New Jersey Transit operates the River LINE from Camden, NJ to Trenton, NJ, every 15 minutes during peak hours and every 30 minutes at other times. Trains are one or two cars. The line is classified Light Rail because it utilizes imported European DMUs that do not meet FRA crash guidelines. The cars may not operate with the Freight rail service that shares the line so evening operating hours are restricted to Saturday nights. This line currently carries over 7,500 passengers on a typical weekday, exceeding expectations.

NCTD operates the Sprinter (passenger rail) line using DMUs built by Siemens AG. Opened March, 2008, The line operates half hourly (hourly on weekends). The line runs from from [[Oceanside, CA), where transfer is possible with Coaster commuter rail service to San Diego, to Escondido, CA. Like the New Jersey Transit River LINE, it is classified Light Rail due to European DMUs, but does not run at a more typical light rail frequency.

Boston's Massachusetts Bay Transportation Authority is reportedly considering options to utilize DMUs on the Fairmount Line, currently run by locomotive-pulled carriages, in order to improve service and headways on the line. This plan has become known as the Indigo Line.

Canada generally follows similar buff strength requirements to the USA, but new services are evaluated on a case-by-case basis. As a result several types of lightweight DMUs have been used:

The O-Train in Ottawa, Ontario uses European-standard Bombardier Transportation Talent DMUs on conventional railway tracks under a specific safety agreement with Transport Canada.

Calgary, Alberta has also used European Sprinter DMUs in demonstration service on conventional railway tracks.

South Korea

Korail operates many DMUs. The DHC (Diesel Hydraulic Car), which made its debut for the 1988 Seoul Olympics, can reach speeds up to 150 km/h and serves Saemaul-ho trains. The NDC (New Diesel Car) serves Mugunghwa-ho trains and the CDC (Commute Diesel Car) serves Tonggeun trains. The NDC and CDC can reach speeds up to 100 km/h and are not used for mountainous lines such as the Taebaek Line.

United Kingdom

The first significant use of DMUs in the United Kingdom was by the Great Western Railway, which introduced its small but successful series of diesel-mechanical GWR railcars in 1934. The London, Midland and Scottish Railway also experimented with DMUs in the 1930s, both on its own system, and on that of its Northern Irish subsidiary, but development was curtailed by World War II.

After nationalisation, British Railways revived the concept in the early 1950s. At that time there was an urgent need to move away from expensive steam traction which led to many experimental designs using diesel propulsion and multiple units. The early DMUs proved successful, and under BR's 1955 Modernisation Plan the building of a large fleet was authorised. These BR 'First Generation' DMUs were built between 1956 and 1963, and some are still in service as of 2008. Most were diesel-mechanical, but a few designs had hydraulic transmissions (these were generally less successful and were withdrawn earlier than the main diesel-mechanical types).

BR's owners, the British Government, required that contracts for the design and manufacture of new locomotives and rolling stock be split between numerous private firms as well as BR's own workshops, while different BR Regions laid down different specifications. The result was a multitude of different types, some of which were built only in small numbers, but in general the First Generation units fell into five distinct groups:

'Low Density' units (such as Class 101), designed for use on rural lines and provincial commuter services. These had 3+2 seating in standard class, and had doors located in vestibules separated from the seats. They were initially allocated to all BR regions except the Western and Southern Regions;
'High Density' units (such as Class 121), designed for the busy commuter services into London, and also for local services in Birmingham, Liverpool and throughout the BR Western Region. These were built with doors at each seating bay, and bench seats across the full width of the carriage for maximum seating capacity. Many were later rebuilt with 3+2 seating and gangways between carriages;
'Cross Country' units, which were essentially more luxurious Low Density units, with 2+2 seating in standard class. They were designed for longer distance rural services, and were originally allocated to the Western and Scottish Regions;
'Intercity' units, which were more substantially constructed, and shared many features with contemporary hauled coaching stock. They were built for express services on important secondary routes on the Scottish, North Eastern and Western Regions.
Parcels cars. Those built as such were invariably single cars with a cab at each end, and were built for the Western and London Midland Regions. Later some passenger DMUs were also rebuilt as parcels units.

There were also a small number of 4-wheeled railbuses built for the most lightly-used branch lines, but these failed to prevent the closure of such lines, and all of the railbuses had gone by the end of the '60s.

Diesel Electric Multiple Units (DEMUs) were also developed during the 1950s and '60s, for use on the Southern Region. The Southern was largely electrified, but required diesel units for the remaining lines, in some cases as stop-gaps pending planned electrification. Diesel-mechanical and diesel-hydraulic units were judged to have inadequate acceleration, which would have caused delays to other traffic when operated over electrified lines. Examples of Southern DEMUs included Classes 205 and 207, which were nick-named "Thumpers" because of their characteristic sound.

At the end of the 1950s, British Railways introduced its Blue Pullman high speed DEMUs. These were few in number and relatively short-lived, but they paved the way for the very successful British Rail "InterCity 125" or High Speed Train (HST) units, which were built between 1975 and 1984 to take over most principal express services on non-electrified routes. These 125mph trains run with a streamlined power car at each end and (typically) 7 or 8 intermediate trailer cars. Although originally classified as DEMUs, the trailer cars are very similar to loco-hauled stock, and the power cars were later reclassified as locomotives under Class 43. They remain in widespread use.

By the early 1980s, many of the surviving First Generation units were becoming life-expired, which lead to spiralling maintenance costs, poor reliability and a poor public image for the railway. A stopgap solution was to convert some services back to locomotive haulage, as spare locomotives and hauled coaching stock were available, but this also increased operating costs. Commencing in the mid '80s, British Rail embarked upon its so called "Sprinterisation" programme, to replace most of the first generation DMUs and many locomotive-hauled trains with three new families of DMU:

Class 140-144 Pacer railbuses, ultra-low-cost diesel-mechanical units utilising 4-wheeled chassis and lightweight bus bodywork, designed for provincial branch line and stopping services. The earliest examples have now been withdrawn from service;
Sprinter a family of diesel-hydraulic DMUs based on the Mark 3 carriage bodyshell. These fall into three sub-groups; Class 150 Sprinters (for branch line/commuter service), Class 153 / 155 / 156 Super Sprinters (for longer cross country services), and Class 158 / 159 Express units (for secondary express services);
Networker diesel-hydraulic units, of Class 165 Network Turbo (standard commuter version) and Class 166 Network Express (for longer distance commuter services). These took over the remaining non-electric commuter services into London.

Following the privatisation of British Rail in the late '90s, several other diesel-hydraulic DMU families have been introduced:

Class 168 Clubman and Class 170/171 Turbostar units, a development of the earlier Networkers. These are built by Bombardier Transportation and are the most numerous and widespread of the post-privatisation designs. The same basic design has been adapted for a variety of different types of use, ranging from commuter to secondary express services;
Class 175 Coradia were designed by Alstom as a rival to the 170 Turbostar, but bought only by First North Western;
Class 180 Adelante were an Alstom design for express services built only for First Great Western;
Class 185 Desiro, a Siemens design introduced to the UK in 2007 for First TransPennine Express services.

As modern diesel-hydraulic units have sufficient performance to match the acceleration of Electric Multiple Units, they have replaced DEMUs on the former Southern Region local/commuter services. However the vast majority of British non-electric InterCity services are currently operated by Diesel Electric Multiple Units, the HSTs having been joined since privatisation by high speed Bombardier Class 220 Voyager, Class 221 Super-Voyager and Class 222 Meridian/Pioneer express units.